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Hollow core cuts Raman thresholds

10 Oct 2002

In today's Science, scientists reveal how a low-loss photonic-crystal fiber could transform gas-based nonlinear optics.

UK scientists have reduced the threshold power needed for stimulated Raman scattering (SRS) by two orders of magnitude. The Bath University team says the breakthrough, which exploits a low-loss, hollow-core photonic-crystal fiber (HC-PCF) filled with hydrogen, marks the beginning of a new era in gas-based nonlinear optics. (Science 298 402)

In SRS, incoming photons interact with hydrogen resulting in a frequency down-converted (Stokes) and an up-converted (anti-Stokes) photon being emitted. Team member Fetah Benabid says the group has set new low-threshold records for both Stokes and anti-Stokes conversion.

The new figures are 800±200 nJ for Stokes conversion and 3.4±0.7 µJ for anti-Stokes conversion.

Benabid says the dramatic reduction is due to the increased interaction length between the pump laser and the Raman active medium [hydrogen]. "In the case of free-space, the interaction length was limited to the only a few millimeters before the laser beam diffracted," he explains.

"People have used capillary fibers but the losses were so high that pieces no longer than one or two centimeters could be used. All these limitations vanish if you use HC-PCF. Light is confined in a hollow core and, with losses less than 1 dB/m, it is contained over much longer lengths."

The researchers fabricate their PCF using a capillary-stacking technique. Seven missing capillaries result in a 15 µm hollow core, which is then filled with hydrogen.

Using a x4 objective lens, 6 ns pulses emitted from a Q-switched Nd:YAG operating at 532 nm are coupled into the fiber's core. The light is then passed into a Raman amplifier. On exit, filtered photodetectors measure the Stokes shift at a wavelength of 683 nm and the anti-Stokes shift at 435.2 nm.

According to Benabid, the low-loss fiber also has a very broad transmission window. "The fiber transmits over more than 700 nm through the visible and IR," he told Optics.org. "This allows effective guidance of the pump and the different Raman components."

The team are now trying to develop fiber with a much lower loss. Benabid says this would allow a low-power CW laser, such as a diode laser, to act as the pump source.

Fibers filled with hydrogen, nitrogen or methane could yield sources at previously unattainable wavelengths in IR, visible and UV regions, says Benabid.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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